1. Field of the Invention
The present invention relates to a signal processing method and a device for the same. The signal processing method is useful for detecting liquid or the like adhering to a surface of a product being monitored.
2. Description of the Related Art
The following describe the prior art by referring to signal processing in a rain sensing method.
Conventionally, many types of rain sensors for vehicles have been developed. For example, a rain sensor employing an optical method as follows is known. A light emitted from a photo emission element (light emission element) is reflected on a sensing surface and the reflected light is received by a photo detector, and then a rain drop is sensed. That is, the reflection condition varies if an object such as a water drop exists on the sensing surface, and the amount of the detected light by the photo detector will decrease. A conventional rain sensor senses a rain drop by detecting this variation.
In the above mention detection, normally, a method for comparison with the reference value (threshold method) had been used (i.e. JP 10-186059 A).
In practical use of such a conventional rain sensor, the rain sensor will be used in various conditions, so that a means for preventing malfunction is necessary. In order to achieve this object, the conventional rain sensor uses plural reference values that are set according to the operation modes (JP 10-186059 A), or replaces and switches reference values one by one (JP 2-68248 A).
As mentioned above, with the conventional rain sensor, the logic for rain drop detection is complicated, and consequently, a high-speed processing for rain drop detection becomes difficult. In addition, in either of the above mentioned conventional methods, the rain drop is detected by the comparison with the reference value based on judging the status on the sensing surface. Therefore, the prevention the malfunction is difficult because of the influence of the light from the outside or the influence of the status of the sensing surface, e.g. where dirt is present.
Moreover, the photo emission element and the photo detector have a disadvantage that the element characteristic will vary when the temperature varies. Especially, the photo emission element such as LED has a characteristic that the output will decrease when the temperature rises. Due to this characteristic, such element has a problem that an appropriate detection cannot be processed unless data correction by monitoring and feeding back the output is processed.
For example, in JP 57-118952 A, a windshield wiper controller is disclosed. This apparatus employs a method in which a light emitted from a photo emission element and reflected on the surface of a windshield glass is received by a photo detector, then the amount of the rain drop is judged according to the received light signal level, and the windshield wiper is driven.
In more detail, the received light signal is detected, then the detected signal is provided to the differential circuit and pulse signals corresponding to the envelope of the received light signal are picked out, and these pulse signals are counted.
In this JP 57-118952 A, the detailed method of how to pick out the pulse signals is not clearly disclosed. However, judging from FIG. 3(c) of the publication, the system will pick out a pulse signal when detecting the intersection of the envelope and the threshold.
In short, in the technology disclosed in the JP 57-118952 A, a rain drop impacting the surface of the windshield may be detected and counted and the detection for impact of a rain drop is processed by comparison of the envelope with the threshold.